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United States Patent |
5,518,778
|
Solovjeva
,   et al.
|
May 21, 1996
|
Single-layer high temperature coating on a ceramic substrate and its
production
Abstract
The single-layer high temperature coating according to the invention, which
is applied in particular on a porous substrate, comprises a high silica
glaze gel frit, alumina and an emissivity agent consisting of MoSi.sub.2
or Cr.sub.2 O.sub.3, and has the following composition by weight:
gel frit: 45 to 55%;
MoSi.sub.2 or Cr.sub.2 O.sub.3 : 25 to 50%;
Al.sub.2 O.sub.3 : 5 to 20%.
Inventors:
|
Solovjeva; Galina A. (Moscow, RU);
Tjurin; Vladimir M. (Moscow, RU);
Solntsev; Stanislav S. (Moscow, RU)
|
Assignee:
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Aerospatiale Societe Nationale Industrielle (FR);
VIAM (RU)
|
Appl. No.:
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404918 |
Filed:
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March 16, 1995 |
Foreign Application Priority Data
Current U.S. Class: |
427/376.2; 427/427 |
Intern'l Class: |
B05D 003/02 |
Field of Search: |
427/376.2,427
|
References Cited
U.S. Patent Documents
3503801 | Mar., 1970 | Huong et al. | 117/221.
|
3653862 | Jan., 1970 | Lynch | 65/30.
|
3953646 | Apr., 1976 | Fletcher et al. | 428/332.
|
3955034 | May., 1976 | Fletcher et al. | 428/332.
|
4093771 | Jun., 1978 | Goldstein et al. | 428/312.
|
4959330 | Sep., 1990 | Donohue et al. | 501/8.
|
5137848 | Aug., 1992 | Barker et al. | 501/18.
|
Primary Examiner: Utech; Benjamin
Attorney, Agent or Firm: Burns, Doane, Swecker & Mathis
Claims
We claim:
1. A process for coating a ceramic substrate, said process comprising the
steps of:
(i) preparing a slurry from a powder comprising 45 to 55 weight % of silica
frit, said silica frit having a high silica content and prepared by using
sol-gel technology, 25 to 50 weight % of an emissivity agent selected from
MoSi.sub.2 or Cr.sub.2 O.sub.3, and 5 to 20 weight % of alumina, in a
dispersion medium, with a weight ratio of powder to dispersion medium of
from 1:1 to 1:5;
(ii) applying said slurry, by spraying under pressure, to a ceramic
substrate which has been dedusted to form a coating layer of said slurry;
(iii) drying said coating layer; and
(iv) firing said coating layer at a temperature of 1300.degree. to
1500.degree. C., for 15 to 90 minutes.
2. The process according to claim 1, wherein the dispersion medium is
distilled water.
3. The process according to claim 1, wherein the ceramic substrate
comprises one or more compounds selected from the group consisting of
Al.sub.2 O.sub.3, SiO.sub.2, ZrO.sub.2, SiC, and Si.sub.3 N.sub.4.
4. The process according to claim 1, wherein approximately 60 to
approximately 80 weight % of particles in the emissivity agent have a size
of less than 5 .mu.m.
5. The process according to claim 1, wherein the coating layer is applied
using compressed air at a pressure of 0.8.multidot.10.sup.5 to
1.1.multidot.10.sup.5 Pa.
6. The process according to claim 1, wherein said ceramic substrate is
porous.
7. The process according to claim 1, wherein said silica frit comprises
from 88 to 98 weight % of SiO.sub.2 and from 2 to 12 weight % of B.sub.2
O.sub.3.
8. The process according to claim 7, wherein said silica frit further
comprises from about 1.multidot.10.sup.-4 to about 1.multidot.10.sup.-5
weight % of Fe.sub.2 O.sub.3 and from about 1.multidot.10.sup.-4 to about
1.multidot.10.sup.-5 weight % of Na.sub.2 O.
9. The process according to claim 1, wherein said coating layer is a
densified layer having a thickness of 70 to 140 .mu.m and a maximum
density of 0.8 g/cm.sup.3.
Description
FIELD OF THE INVENTION
The invention relates to materials, and more precisely to coatings, for the
protection of ceramic substrates, in particular porous ones, from erosion,
and chemical and mechanical failure.
Refractory oxide based fibrous ceramic materials functioning at high
temperatures are widely used. The conditions under which these materials
are used place demands on the coatings used for their protection: high
heat resistance, erosion resistance, thermochemical stability and phase
stability.
BACKGROUND OF THE INVENTION
There is known at present a whole range of erosion resistant coatings which
operate at temperatures of up to 1260.degree. C. There is known a
two-layer coating comprising a barrier layer and a glaze layer (see U.S.
Pat. No. 3,953,646). The barrier layer is formed by a slip of fused silica
comprising approximately 80 to 90% by weight of solid material. The
coating is applied to the substrate by spraying. The barrier layer is
fired at a temperature of approximately 930 to approximately 1370.degree.
C. The glaze layer, consisting of high silica glass, of borosilicate glass
and of an emissivity agent, is applied to the barrier layer. The
emissivity agent is selected from the group formed by silicon carbide,
chromium, cobalt and nickel oxides, nickel-chromium spinels, silicon
nitride and calcined, mixed oxides of iron, chromium and/or nickel. High
silica glass (Corning Glass No. 7913) contains not less than 94 weight %
of SiO.sub.2. The weight composition of the borosilicate glass (Corning
Glass No. 7740) is as follows: 70 to 87% of SiO.sub.2 , 10 to 20% of
B.sub.2 O.sub.3 2 to 5% of Na.sub.2 O and 1 to 5% of Al.sub.2 O.sub.3.
The high silica glass component and the borosilicate glass component are
used in a weight ratio of approximately 3:1 to approximately 19:1, and the
glass components (high silica glass and borosilicate glass) and the
emissivity agent are used in a weight ratio ranging from 50:1 to
approximately 4:1. An aqueous slurry containing from approximately 10 to
approximately 90 weight % of glaze coating is fired at a temperature
ranging from 930 to approximately 1370.degree. C.
There is known in the art (see U.S. Pat. No. 3,955,034) a three-component
coating for silica insulation comprising a silica barrier layer, an
emissivity layer comprising a high silica glass component and an
emissivity agent selected from the group formed by silicon carbide, nickel
oxide, chromium oxide, cobalt oxide, a nickel-chromium spinel, silicon
nitride and calcined, mixed oxides of iron, chromium and cobalt, with a
weight ratio of the high silica glass to the emissivity agent ranging from
approximately 50:1 to approximately 4:1, and an overglaze coating layer of
high silica glass and borosilicate glass in a weight ratio of high silica
glass to borosilicate glass ranging from approximately 3:1 to
approximately 19:1. The coating is fired at a temperature ranging from 930
to approximately 1370.degree. C.
These coatings provide neither sufficient thermal shock resistance nor
sufficient heat emissive stability, and they undergo a shrinkage.
To overcome the problems mentioned above, there has been proposed a
one-layer coating (see U.S. Pat. No. 4,093,771) which is prepared by
reacting a compound, selected from the group of substances formed by
silicon tetraboride, silicon hexaboride, other boron silicides, boron and
mixtures of these substances, with a reactive glass frit composed of high
silica porous borosilicate glass and boron oxide. A thin layer of
borosilicate glass is formed on finely divided particles of high silica
glass, which improves the sintering of the coating without a substantial
increase in the thermal expansion coefficient.
The reactive glass frit is advantageously prepared by blending
approximately 2 to 10 parts by weight of boron oxide with 100 parts by
weight of high silica porous borosilicate glass, such as Vycon.RTM. 7930
glass. Vycon.RTM. 7930 high silica borosilicate glass has a porosity of
approximately 28%. The boron oxide is dissolved in 200 to 400 parts by
weight of deionized water. The mixture is stirred at approximately
95.degree. C., and then dried for a period of up to 24 hours, at a
temperature of 75.degree. to 95.degree. C. The resulting glass frit is
dispersed, screened and fired at 1150.degree. C. for 1 hour. The resulting
sintered composite is ground to a powder and screened.
A typical composition would be 97.5 weight % of reactive glass frit
containing 5.5 weight % of boron oxide, combined with 2.5 weight % of
silicon tetraboride composed of 63.+-.3 weight % of silicon, 36.+-.3
weight % of boron and less than 0.2 weight % of magnesium. The coating
slurry is prepared by blending finely divided particles of reactive glass
frit and silicon tetraboride, with a carrier such as ethanol and a
pre-binder such as methylcellulose, in a proportion by weight of solid
components of 35 to 50%. The mixture of coating components is milled in an
alumina ball mill with alumina balls for 3 to 12 hours. The coating is
applied by spraying. The coated samples are dried for 2 to 5 hours at
temperatures in the range of 20 to approximately 70.degree. C. After
drying, the coated samples are glazed in an oven for 1.5 hours at
1215.degree. C. The coating has an emissivity of approximately 0.90 to
0.93 from ambient temperature to over 1260.degree. C. The thermal
expansion coefficient is 1.1.multidot.10.sup.-6 K.sup.-1.
There is also known an advanced low density coating for the protection of
aluminosilicate porous materials that has an operating temperature of up
to 1300.degree. C. The composition of the coating comprises 77.5 weight %
of reactive glass frit, 2.5 weight % of silicon tetraboride and 20 weight
% of molybdenum disilicide. The coating is formed on the substrate at
1230.degree. C. for 1.5 hours (see: Advanced Porous Coating for low
density Ceramic Insulation Materials, J. Amer. Ceram. Soc., vol. 72, No.
6, pages 1003-1010, 1989).
There is further known a coating on an insulating ceramic material
comprising 80 to 95 weight % of aluminosilicate glass and 5 to 20 weight %
of aluminum oxide. This coating is formed using slurry coating and firing
techniques. It is fired at temperatures ranging from 1300.degree. to
1350.degree. C. for 5 to 15 minutes. At temperatures of up to 1300.degree.
C., the coating has low erosion resistance and tends to crack [see
Inventor's Certificate SU 1 331 846 (classification: C 03 c 8/24) filed on
21st. Jun. 1985, published on 23rd. Aug. 1987 (Bulletin No. 31) and
entitled "Coating Composition"].
Known coating compositions are used for the protection of porous ceramic
materials having operating temperatures of 1260.degree. C. to 1350.degree.
C. At present, ceramic materials with operating temperatures of up to
1500.degree. C. are a matter of interest for specialists. Known coating
compositions are not efficient at these temperatures: the coatings
obtained are crystallised and cracked.
SUMMARY OF THE INVENTION
According to one of its aspects, the invention provides a single-layer,
high temperature coating on a ceramic substrate, in particular a porous
one, characterized in that it comprises a high silica glaze gel frit,
alumina and an emissivity agent consisting of MoSi.sub.2 or Cr.sub.2
O.sub.3, and has the following weight composition:
gel frit: 45 to 55%;
MoSi.sub.2 or Cr.sub.2 O.sub.3 : 25 to 50%;
Al.sub.2 O.sub.3 : 5 to 20%.
The ceramic material used as a substrate generally comprises one or more
compounds selected from the group formed by Al.sub.2 O.sub.3, SiO.sub.2,
ZrO.sub.2, SiC and Si.sub.3 N.sub.4.
The high silica glaze gel frit is prepared using a sol-gel technology and
is preferably of the following weight composition:
SiO.sub.2 : 88 to 98%;
B.sub.2 O.sub.3 : 2 to 12%.
The Fe.sub.2 O.sub.3 content and the Na.sub.2 O content of the gel frit
generally range from approximately 1.10.sup.-4 to approximately
1.10.sup.-5 weight %.
These Fe.sub.2 O.sub.3 and Na.sub.2 O contents entering into the
composition of the gel frit ensure that it has a high softening
temperature and low crystallability, which enables the gel frit to be used
as a matrix for a coating having an operating temperature of up to
1500.degree. C.
The use of less than 45 weight % of gel frit has the result of lowering the
softening temperature and the thermal shock resistance of the coating.
A coating comprising more than 55 weight % of gel frit does not ensure
sufficient emissivity.
The use of alumina (Al.sub.2 O.sub.3) increases the heat resistance of the
coating.
The use of MoSi.sub.2 or Cr.sub.2 O.sub.3 results in high emissivity at a
temperature of 1500.degree. C. Moreover, the addition of these components
to a coating composition has the effect of increasing the thermal
expansion coefficient and reducing the residual strain in the coating,
which makes it possible to improve its heat resistance. The use of over 50
weight % of these additives has the effect of reducing resistance of the
coating to thermal shocks, and the use of less than 25 weight % of these
additives has the effect of reducing the emissivity of the coating.
Advantageously, the emissivity agent content in particles having a size of
less than 5 .mu.m ranges from approximately 60 to approximately 80 weight
% of this agent. If the emissivity agent content in particles having a
size of less than 5 .mu.m is over 80 weight % or less than 60 weight % the
agent is randomly distributed through the glass matrix, which results in a
drop in the heat resistance of the coating.
Penetration of the substrate, in particular a porous one, by the coating
imparts high adhesion.
In general a densified layer having a thickness of 70 to 140 .mu.m and
maximum density of 800 kg/m.sup.3 (0.8 g/cm.sup.3) is formed on the
substrate. The coating is applied to the substrate by spraying, using
compressed air, advantageously at a pressure of 0.8.multidot.10.sup.5 to
1.1.multidot.10.sup.5 Pa (0.8 to 1.1 atm).
The weight ratio of the suspension phase (coating powder) to the dispersion
medium (preferably distilled water) is from 1:1 to 1:5. A high content in
terms of dispersion medium, particularly water, results in the chemical
composition of the coating being non-uniform, while a low content for this
medium leads to reduced adhesion between the coating and the substrate.
The coating is applied to the surface of the substrate, prepared in advance
by dedusting the felt forming the substrate so as to obtain better
adhesion.
According to another of its aspects, the invention thus relates to a
process for providing a ceramic substrate, in particular a porous one,
with a coating as defined hereabove, characterized in that it essentially
comprises the steps of:
preparing a slurry from a powder consisting of 45 to 55 weight % of high
silica glaze gel frit, 25 to 50 weight % of emissivity agent, MoSi.sub.2
or Cr.sub.2 O.sub.3 and 5 to 20 weight % of alumina, in a compatible
dispersion medium, preferably distilled water, with a weight ratio of
powder to dispersion medium of from 1:1 to 1:5;
applying this slurry, by spraying under pressure, to the ceramic substrate
to be coated, which has undergone a preparatory treatment;
drying the layer thus obtained and firing it at a temperature of
1300.degree. to 1500.degree. C., for 15 to 90 minutes.
Drying is generally carried out at approximately 20.degree. C. for
approximately 30 minutes, and then at a temperature of 80.degree. to
100.degree. C. for approximately 2 hours.
The following examples are intended to illustrate and more clearly explain
the invention.
EXAMPLE 1
A coating was prepared using slurry coating and firing techniques.
The gel frit used was milled in an alumina mill with alumina balls for 25
hours, to obtain a powder having a specific surface of 1 m.sup.2 /g, and
screened. MoSi.sub.2 powder was milled in a metallic mill with metallic
balls containing isopropyl alcohol for 30 hours, and then screened.
45 parts by weight of frit (comprising 88 weight % of SiO.sub.2 and
1.10.sup.-4 weight % of Fe.sub.2 O.sub.3 and Na.sub.2 O), 50 parts by
weight of MoSi.sub.2 (comprising 65 weight % of particles having a size of
less than 5 .mu.m) and 5 parts by weight of Al.sub.2 O.sub.3 were mixed in
a polyethylene vessel for 40 hours.
A weighed quantity of powder was diluted with distilled water in a weight
ratio of 1:2. The coating was o applied by spraying, at an air pressure of
0.8.multidot.10.sup.5 Pa (0.8 atm), on the surface prepared in advance, by
dedusting, to receive a coating of an Al.sub.2 O.sub.3 based material. A
70 .mu.m thick densified layer was formed. The coating was air dried at
20.degree. C. for 30 minutes, and then oven dried at 80.degree. C. for 2
hours. The coating was fired at 1300.degree. C. for 90 minutes.
EXAMPLE 2
A coating comprising 45 parts by weight of high silica gel frit containing
12 weight % of B.sub.2 O.sub.3, 50 parts by weight of Cr.sub.2 O.sub.3 and
5 parts by weight of Al.sub.2 O.sub.3 was applied, using the process
according to Example 1, to a ZrO.sub.2 based substrate. The powder:water
weight ratio was 1:5, and the air pressure was 1.1.multidot.10.sup.5 Pa
(1.1. atm). A 140 .mu.m thick densified layer was deposited.
EXAMPLE 3
A coating comprising 55 parts by weight of high silica gel frit (containing
98 weight % of SiO.sub.2 and 1.10.sup.-5 weight % of Fe.sub.2 O.sub.3,
Na.sub.2 O), 30 parts by weight of MoSi.sub.2 (containing 75 weight % of
particles having a size of less than 5 .mu.m) and 15 parts by weight of
Al.sub.2 O.sub.3 was applied, according to the process of Example 1, to an
SiO.sub.2 based substrate. The powder:water weight ratio was 1:3, and the
air pressure was 1.10.sup.5 Pa (1 atm). A 100 .mu.m thick densified layer
was deposited on the substrate. The coating was fired at a temperature of
1500.degree. C. for 15 minutes.
RESULTS
The coating compositions obtained were subjected to a heat resistance test
over a temperature range of 20.degree. C..revreaction.1500.degree.
C..revreaction.20.degree. C. for 30 minutes. The number of cycles was 2.
The thermal expansion coefficient was 2.3.multidot.10.sup.-6 K.sup.-1, and
the emissivity was not less than 0.86.
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